Some sensors such as the photo transistors produce a current which needs to be converted into a digital value. Conventional photo smoke sensors require external circuitry to handle the often very small output currents therefrom. For example, present technology photo smoke detectors use high speed amplifiers to detect the light change in the smoke chamber. A current technology interface for a current to voltage conversion use transconductance amplifiers which require an operational amplifier. Operational amplifiers may also be used to control the excitation current for a light emitting diode (LED) in a smoke detection photo chamber.
There are three types of smoke detectors available today: (1) photoelectric, (2) ionization, and (3) a combination of photoelectric and ionization that have both types of sensors. A photoelectric alarm is triggered when smoke detected based upon the amount of light detected from a light source onto a light sensor. In an ion smoke detector, ionized air molecules attach to the smoke particles that enter the chamber, reducing the ionizing current and triggering the smoke alarm detection circuits.
The ion detector reacts faster than the photoelectric detector in responding to flaming fires, and the photoelectric detector is more responsive to smoldering fires. Because an ion detector tests the air for small combustible particles, it can be fooled by chemical or paint particles in the atmosphere. The photoelectric detector, which needs to “see” the smoke from the fire, can be fooled by dust, steam or even spider webs. Though both offer protection against undetected fires, ion detectors experience a higher incidence of nuisance alarms.
Optical beam smoke detectors work on the principle of light obscuration, where the presence of smoke blocks some of the light from the light source beam. Once a certain percentage of the transmitted light has been blocked by the smoke, a fire alarm is generated. Optical beam smoke detectors are typically used to detect fires in large commercial and industrial buildings, as components in a larger fire alarm system.
Optical beam smoke detectors consist of at least one light transmitter and one receiver, which is photosensitive. The photosensitive receiver monitors light produced by the transmitter under normal conditions. In the absence of smoke, light passes from the light transmitter to the receiver in a straight line. In a fire, when smoke falls within the path of the beam detector, some of the light is absorbed or scattered by the smoke particles. This creates a decrease in the received light signal, leading to an increase in optical obscuration i.e. a reduction of transmittance of light across the beam path.
It would be desirable to improve the reliability, lower the power consumption and reduce manufacturing costs of fire alarms using photo optical smoke detectors.